1. Field of the Invention
The present invention relates to a high voltage generation apparatus which is used for an image formation apparatus.
2. Related Background Art
A high-voltage power supply circuit has been installed in an image formation apparatus which applies an electrophotographic system, and is indispensable to an image formation process on a paper or the like. As the high-voltage power supply circuits, there are various modularized power supplies, e.g., a charge high-voltage power supply, a development high-voltage power supply, a transfer high-voltage power supply, a fixing high-voltage power supply and the like. Each of such high-voltage modules has a different specification according to the various structure of the image formation apparatus, e.g., the structure in which an AC power supply is superimposed on a DC power supply, the structure in which a DC plus power supply is superimposed on a DC minus power supply, or the like. Also, there are various specifications on a specified voltage and a specified current, a constant current control system and a constant voltage control system, a single-value output and a multistage-value control output, a load condition, and the like. In such cases, it is indispensable to use a constant voltage control circuit and a constant current control circuit such that constant voltage and current can be outputted in various conditions.
Ordinarily, a voltage detection circuit is installed in the constant voltage control circuit, and a current detection circuit is installed in the constant current control circuit. However, it can be considered a case where both the voltage detection circuit and the current detection circuit are installed in the constant voltage control circuit, and thus a constant voltage control operation is performed as a current value is monitored. Also, it can be considered a case where both the constant voltage control circuit and the constant current control circuit or both the voltage detection circuit and the current detection circuit are provided to perform a constant current control operation, a voltage value in the operation is monitored, and a calculation process is performed by using the monitored voltage value to perform the constant voltage control operation. The reason why such the cases (or methods) have been considered is to solve following problems. That is, in a case where a bias is applied only based on the constant voltage control operation, a resistance value of a transfer roller or the like is highly changed due to environment, especially humidity. Thus, since a transfer current is also changed, defective transfer comes to frequently occur. Further, in a case where the bias is applied only based on the constant current control operation, if the size (width) of a transfer member which passes over the transfer roller is small, both an area in which the transfer member exists on the transfer roller and an area in which the transfer member does not exist on the transfer roller are managed as output loads for the transfer bias. Since impedance of the area in which the transfer member does not exist on the roller is lower than that of the area in which the transfer member exists, the current flows in the area in which the transfer member does not exist. Thus, since the current does not sufficiently flow in the area in which the transfer member exists on the roller, the defective transfer comes to frequently occur at such the area.
FIG. 5 is a block circuit diagram showing the schematic structure of a high voltage generation circuit applying a conventional constant voltage control system (conventional example 1).
In FIG. 5, a high voltage control circuit is composed of a step-up transformer 101, a switching unit 102, a high-voltage rectifier diode 103, a high-voltage capacitor 104, a constant voltage control unit 105, a voltage detection unit 106 and a resistor 107.
In this structure, the switching unit 102 performs switching driving on the step-up transformer 101 in a predetermined frequency and a predetermined duty ratio. The step-up transformer 101 driven in a predetermined input voltage by the switching unit 102 boosts the input voltage to generate a high voltage of a predetermined pulsating waveform. Then, when the pulsating-waveform high voltage generated by the step-up transformer 101 is rectified and smoothed respectively by the high-voltage rectifier diode 103 and the high-voltage capacitor 104, a DC high voltage is generated and supplied to a load 110. The output voltage of the load 110 is always monitored by the voltage detection unit 106.
The voltage detection unit 106 contains therein a high-resistance bleeder resistor 107 for discharging an electric charge refreshed (or charged) in the high-voltage capacitor 104 by the step-up transformer 101 and the high-voltage rectifier diode 103. Namely, the unit 106 is structured to convert the high-voltage output voltage into a low-voltage detection signal level. The constant voltage control unit 105 always monitors a detection signal at the voltage detection unit 106, and controls the switching unit 102 such that the high-voltage output voltage generated from the step-up transformer 101 has a desired value. By structuring as above, according to the conventional example 1 shown in FIG. 5, a high-voltage power supply of the constant voltage control system capable of outputting the desired output voltage under various conditions can be obtained.
FIG. 6 is a block circuit diagram showing the schematic structure of an another high voltage generation circuit applying a conventional constant current control system (conventional example 2).
In FIG. 6, a high voltage control circuit is composed of a step-up transformer 111, a switching unit 112, a high-voltage rectifier diode 113, a high-voltage capacitor 114, a bleeder resistor 117, a constant current control unit 118 and a current detection unit 119.
In the above structure, the step-up transformer 111, the switching unit 112, the high-voltage rectifier diode 113, the high-voltage capacitor 114 and the bleeder resistor 117 have the same functions as those of the corresponding elements in the high voltage generation circuit shown as the conventional example 1. A DC current flowing in a load 120 formed at an output unit forms a current loop such as a route A shown, and such the load current is detected by the current detection unit 119.
The constant current control unit 118 always monitors a load current value detected by the current detection unit 119, and controls the switching unit 112 such that the current flowed from the high-voltage output unit in the load 120 has a desired value.
By structuring as above, according to the conventional example 2 shown in FIG. 6, a high-voltage power supply of the constant current control system capable of outputting the desired output current under various conditions can be obtained.
Further, as the high voltage generation circuits in the conventional examples 1 and 2, there are various circuits which are structured to be able to output a multistage control voltage and a multistage control current by variably changing the constant voltage control value and the constant current control value respectively used in the constant voltage control unit 105 and the constant current control unit 118.
However, in recent years, in the above-described high voltage generation circuit of the constant voltage control system or the high voltage generation circuit of the constant current control system, the high voltage generation circuit of the constant voltage control system capable of monitoring the output current value, and the high voltage generation circuit of both the constant current control system and the constant voltage control system capable of monitoring the output voltage value have become necessary.
FIG. 7 is a block circuit diagram showing the schematic structure of a conventional high voltage generation circuit applying both the constant current control system and the constant voltage control system.
In FIG. 7, a high voltage control circuit is mainly composed of a step-up transformer 121, a switching unit 122, a high-voltage rectifier diode 123, a high-voltage capacitor 124, a constant voltage control unit 125, a bleeder resistor 127, a current detection unit 129, a voltage detection unit 126 and a controller 128. In the above structure, the step-up transformer 121, the switching unit 122, the high-voltage rectifier diode 123, the high-voltage capacitor 124, the bleeder resistor 127 and the current detection unit 129 have the same functions as those of the corresponding elements in the high voltage generation circuit shown in FIG. 5 or 6.
The voltage detection unit 126 is composed of a rectifier diode 126a, a capacitor 126b and a resistor 126c. The step-up transformer 121 contains three windings 121a, 121b and 121c. In these windings, the winding 121c is used as a voltage detection winding. Since these windings are magnetically coupled together, a voltage generated at the end of the winding 121b has a voltage value tracking a voltage value at the end of the winding 121c. On the contrary, the voltage generated at the end of the winding 121c has the voltage value tracking the voltage value at the end of the winding 121b. Therefore, when a DC voltage obtained by rectifying and smoothing the output voltage at the end of the winding 121c is monitored by the constant voltage control unit 125 and subjected to constant voltage controlling, also an output voltage at the end of the winding 121b after the rectifying and smoothing can be controlled to be the desired voltage value. A DC current flowing in a load 130 connected to an output unit forms a current loop such as a route B shown, and the current detection unit 129 detects such the load current. The controller 128 always monitors the load current value detected by the current detection unit 129, and properly varies a setting voltage of the constant voltage control unit 125 such that the desired DC current flows in the load 130. By structuring as above, according to a conventional example 3 shown in FIG. 7, a constant-voltage power supply capable of causing the appropriate load current to flow in the load 130 which varies according to environment and the like can be provided.
A high voltage generation circuit which has a means for monitoring the above output voltage value and applies both the constant current control system and the constant voltage control system is illustrated in FIG. 8 (conventional example 4).
FIG. 8 is a block circuit diagram showing the schematic structure of the conventional high voltage generation circuit applying both the constant current control system and the constant voltage control system.
In FIG. 8, a high voltage control circuit is composed of a step-up transformer 131 which has windings 131a, 131b and 131c, a switching unit 132, a high-voltage rectifier diode 133, a high-voltage capacitor 134, a bleeder resistor 137, a constant voltage/constant current bicontrol unit 135, a current detection unit 139, a voltage detection unit 136 which contains a rectifier diode 136a, a capacitor 136b and a resistor 136c, and a controller 138. The step-up transformer 131, the switching unit 132, the high-voltage rectifier diode 133, the high-voltage capacitor 134, the bleeder resistor 137 and the current detection unit 139 have the same functions as those of the corresponding elements in the high voltage generation circuit shown in FIG. 7. The voltage detection unit 136 is connected to the winding 131c in the step-up transformer 131 so as to detect a voltage generated at the winding 131c.
In the above structure, since the three windings 131a, 131b and 131c in the step-up transformer 131 are magnetically coupled together, a voltage generated at the end of the winding 131b has a voltage value tracking a voltage value at the end of the winding 131c. On the contrary, the voltage generated at the end of the winding 131c has the voltage value tracking the voltage value at the end of the winding 131b. Therefore, when a DC voltage obtained by rectifying and smoothing the output voltage at the end of the winding 131c is monitored by the controller 138, a voltage at an output unit can be calculated.
The constant voltage/constant current bicontrol unit 135 is structured to be able to switch constant voltage controlling and constant current controlling in response to a control signal from the controller 138. In order to perform the constant current controlling by using the current detection unit 139, the controller 138 initially sends the signal to the constant voltage/constant current bicontrol unit 135 so as to constant-current drive the high voltage generation circuit. In such the constant-current driving of the high voltage circuit, an output voltage value of this circuit is monitored by the voltage detection unit 136. Subsequently, the controller 138 switches the constant current controlling to the constant voltage controlling such that the detected output voltage value becomes constant. Then, the controller 138 controls the switching unit 132 such that the high-voltage output voltage generated by the step-up transformer 131 has a desired value.
By structuring as above, according to the conventional example 4 shown in FIG. 8, a high-voltage power supply capable of making, in spite of the high-voltage power supply, appropriate a load current which varies according to environment and the like can be provided.
As described above, the step-up transformer 121 in the conventional example 3 contains the three windings including the voltage detection winding 121c, and the windings 121b and 121c are tracking operated. Similarly, the step-up transformer 131 in the conventional example 4 contains the three windings including the voltage detection winding 131c, and the windings 131b and 131c are tracking operated.
However, these windings are merely coupled together magnetically. Further, since the winding 121b or 131b is used to generate the high voltage, it is wound with an extra fine wire by several thousands turns. On the other hand, since the winding 121c or 131c is used to detect the voltage, it is wound with the wire merely by several tens turns. Therefore, an individual error occurs in relative ratio of output values of, e.g., the windings 121b and 121c. This error directly increases an error in detection level. For this reason, in the conventional structure, there has been a problem that it is difficult to detect both the current and the voltage and also accurately detect the output voltage.
As described above, since the accurately detection of the output voltage is difficult in the conventional examples, it has been supposed that, e.g., the bleeder resistor 117 in the conventional example 2 shown in FIG. 6 is replaced with a voltage detection unit to increase voltage detection accuracy.
FIG. 9 is a block diagram showing the schematic structure of such the conventional high voltage generation circuit. In the drawing, a high voltage control circuit is composed of a step-up transformer 141, a switching unit 142, a high-voltage rectifier diode 143, a high-voltage capacitor 144, a voltage detection unit 146, a constant voltage control unit 145, a current detection unit 149 and a controller 148. As shown in the drawing, the voltage detection unit 146 is structured to detect a voltage of an output unit itself. An output voltage value detected by the voltage detection unit 146 is sent to the constant voltage control unit 145. By structuring as above, an output voltage can be accurately detected.
However, even in the high voltage generation circuit shown in FIG. 9, the current detection unit 149 resultingly detects a current obtained by adding currents respectively flowing in routes C and D to each other. In this case, the route C represents a current loop flowing in the voltage detection unit 146, and the route D represents a load current loop. That is, even if the high voltage generation circuit is structured as shown in FIG. 9, there has been a problem that load current detection accuracy is degraded instead of improvement of voltage detection accuracy.